Since years there’s a bug in Inkscape where – after drawing lines – stroke markers always stay black, even if the color of the strokes are changed.
It’s funny to see some bugs never being fixed but this bug is the most annoying I’ve ever seen and which is driving people crazy.
Stroke with end markers
How to fix this
Luckily Inkscape provides an extension which colorizes the end markers with the stroke color.
Just select your stroke and click on Extensions > Modify Path > Color Markers to Match Stroke.
After applying the plugin on the stroke we finally have a stroke with its arrow heads colorized in the same color as the stroke itself.